The invention relates to a process for preparing stable water solutions of sulfonated melamine-formaldehyde and melamine-urea-formaldehyde condensates. The invention also relates to the stable solutions, themselves, and to the use of such stable solutions as superplasticizer additives for mixtures containing inorganic settable cementitious materials, such as Portland cement, magnesia cements, gypsum and the like.
Cementitious materials, such as Portland cement, magnesia cement, gypsum and the like can be dispersed in water to form a settable paste. These materials can be used alone or they can be used in combination with solid fillers, such as sand, stone or the like, which impart significantly improved physical properties to the ultimately set or hardened structures formed therefrom. The mechanism that is responsible for the setting or hardening of the cementitious material is the hydration thereof. Accordingly, it is evident that the water that is mixed with the cementitious and filler serves not only as a means for dispersing the cementitious material and filler so as to impart workability or flowability to the resulting paste, but as the source of the water of hydration for the setting reactions.
The art relating to cementitious materials and concrete is highly developed. As a result, it is well known that, in theory, a water/cement ratio of about 0.27 is adequate for the hydration of cement. It is also well known that any water in excess of that amount that is used in a cementitious mix or concrete to impart increased workability will detract from the potential compressive strength that can be achieved. For instance, the correct selection of cement, aggregate and curing conditions can yield concrete having a compressive strength of 960 kg/cm.sup.2 and a flexural strength greater than 110 kg/cm.sup.2. However, most common concrete placing practices demand a level of workability which cannot normally be obtained without the addition of some, and often considerable water above the theoretical requirement, thereby limiting the physical characteristics of the resulting concrete.
In the past, some improvement in the workability of cementitious pastes and concretes has been achieved by the addition thereto of small amounts of anionic plasticizers such as unmodified lignosulfonates and polyhydroxycarboxylic acids. These anionic plasticizers are believed to be absorbed onto the solid particles of the cementitious material and aggregate, thereby causing them to become mutually repulsive and more readily dispersible in water.
Relatively recently, these anionic plasticizers have been replaced with a variety of materials that have exhibited far superior properties in terms of their ability to aid in the dispersing of the cementitious materials and aggregate. In general, these newer plasticizers, which are referred to as superplasticizers, impart extreme workability to the cementitious paste that is formed and, in addition, allow for the use of considerably less water than do the normal plasticizers (often as much as a 30% or more water reduction for comparable workability).
Among the currently available superplasticizers there may be included resinous materials such as sulfonated melamine-formaldehyde condensates, and modified lignosulfonates. Within each of these known types of superplasticizers variations occur; for example, chemically similar materials may have different molecular weights, a different pH, and so on, which alter their effectiveness, stability and overall usefulness.
One such superplasticizer is disclosed in U.S. Pat. No. 4,444,945 to Sheldrick. In that patent, it is taught to produce a sulfonated melamine-formaldehyde condensation product in two stages. In the first stage, melamine, formaldehyde and a bisulfite are condensed in an aqueous solution at a pH of 8-10 for 1-5 hours. In the second stage, condensation is continued at a pH of 6-7 for 2-6 hours.
Another superplasticizer is disclosed in U.S. Pat. No. 4,272,430 to Pieh et al. However, in accordance with that patent, the superplasticizer is a copolymer of melamine-urea-formaldehyde and an aromatic aminosulfonic acid.
The preparation of sulfonated melamine-formaldehyde resins is also disclosed in U.S. Pat. Nos. 3,985,696, 3,941,734, 3,870,671 and 3,661,829 to Aignesberger et al. In accordance with the first of these patents, the preparation involves a three-stage process. The initial stage comprises condensing melamine or a mixture of melamine and urea with formaldehyde and an alkali metal sulfite in an aqueous solution at 60.degree.-80.degree. C. and a pH of 10-13. In a second stage, the condensation is continued at a pH of 3-4.5; and in a third stage, the temperature is adjusted to 70.degree.-95.degree. C. and the pH is adjusted to 7.5-9 until a sample of the solution, when diluted to 20% solids, has a viscosity of 5-40 centipoises at 20.degree. C.
In the second of these four Aignesberger et al. patents, an initial condensation of an aqueous solution of melamine and formaldehyde preferably is conducted at a pH of 3.5-5, and at a temperature of 75.degree.-85.degree. C. for 20-100 minutes prior to cooling and the addition of an alkali metal sulfite to the reaction medium. After the sulfite is added, the condensation mixture is adjusted to a pH above 7, e.g., between 8 and 11, and refluxed for 1-3 hours. A similar preparation is disclosed in the third and fourth Aignesberger et al. patents.
Other patents which relate to the preparation of sulfonated melamine-formaldehyde condensates in multiple stages include U.S. Pat. No. 2,730,516 to Suen et al. and U.S. Pat. No. 2,407,599 to Auten et al. In accordance with the Suen et al. patent, it is critical that the pH of the reaction mixture be within the range of from about 1.5 to 3.7 during the polymerization of the reactants so that the resulting products have the desired properties. Among the properties that are disclosed is a minimum viscosity of 50 centipoises at 20% solids content at 25.degree. C.
The Auten et al. patent also discloses the use of a relatively low pH in the initial condensation step, i.e., from about 7-10 for melamine-formaldehyde condensates and from about 4-10 for melamine-urea-formaldehyde. The temperature during the first stage condensation is from about 60.degree.-105.degree. C.
While many of the sulfonated melamine-formaldehyde and sulfonated melamine-urea-formaldehyde resin solutions produced in accordance with the known processes exhibit superplasticizing properties, they generally exhibit poor stability, particularly when they are to be used in concrete mixtures in areas characterized by severe climatic conditions, such as those in the Arabian Gulf. In addition, many of the prior art sulfonated superplasticizer resin solutions contain salt impurities, such as sulfates, which, if left in the resin solutions, can have undesirable effects when the superplasticizers are added to cementitious materials. For example, it is well known the salts of various kinds can alter the hydration reaction of Portland cement even when present in low concentrations, thereby harming the mechanical properties of concrete structures formed therefrom.
Still further, the known processes for preparing the sulfonated melamine-formaldehyde and melamine-urea-formaldehyde resin solutions require long cycle times, thereby adversely affecting the economics of the resin solutions so produced.